1. Field of Invention
The invention relates to a range-switching device that selects a transmission range in accordance with need in a vehicular automatic transmission, and to a control method of the range-switching device.
2. Description of Related Art
Generally, the selectable transmission ranges set in a vehicular automatic transmission are a parking range (P), a reverse range (R), a neutral range (N), a drive range (D), etc.
As for a method of selecting one from these transmission ranges and the like, a transmission range is selected by a driver operating a shift lever that is disposed near the driver's seat of the vehicle.
In the drive range D, the speed change ratio is automatically switched to an optimal speed change step (preset ratio) in accordance with the situation of the driving of the vehicle.
An example of the range-switching device for selecting a transmission range is a device having a detent lever that is pivotably supported and that is rotationally driven in an appropriate direction so as to change the state of a manual valve that is a component element of a transmission range-switching hydraulic control device provided in the automatic transmission, and a shift cable for drivingly interconnecting the detent lever and a shift lever.
That is, in such a range-switching device, as the shift lever is operated by a driver of the vehicle, the state of the manual valve is appropriately changed via the shift cable and the detent lever so as to switch the transmission range to the range selected by the driver operating the shift lever.
However, in the system employing the shift cable, there are inconveniences and the like, for example, considerable restrictions in the layout of the shift cable, and the like. Therefore, for example, Japanese Patent Application Publication No. 2001-182827 (JP-A-2001-182827) discloses a system called shift-by-wire.
This technology has a construction in which the shift lever and the detent lever are not interconnected by a shift cable but are mechanically separated, and in which when a driver operates the shift lever, the thus-selected transmission range is detected by a sensor or the like, and the state of the manual valve is appropriately changed by driving the detent lever via an actuator, such as a motor or the like, so as to switch the transmission range to the detected range.
Furthermore, as shown in Japanese Patent Application Publication No. 2001-295922 (JP-A-2001-295922), a related-art range-switching device is provided with a parking lock device for putting an output shaft of the automatic transmission into a locked state in which the output shaft is unrotatable, when the transmission range is switched to the parking range P.
The parking lock device is able to put the output shaft into the locked state in which the output shaft is unrotatable by engaging the claw of a parking lock pawl with a parking gear that is fixed to an outside of the output shaft, and to put the output shaft into an unlocked state in which the output shaft is rotatable by drawing the hook apart from the parking gear.
In the aforementioned art related to the invention, if during a rest of the vehicle, the parking range P is selected through the operation of the shift lever, the transmission range is switched to the parking range P even in the case where the posture of the vehicle is inclined forward or rearward.
However, in the case where the vehicle is inclined, the output shaft of the automatic transmission receives a torque that acts to turn the output shaft (parking load) from the wheels of the vehicle, so that the claw of the parking lock pawl tends to strongly engage with the parking gear.
Therefore, the larger the inclination angle of the vehicle, the larger the torque becomes, and the stronger the engagement of the claw of the parking lock pawl into the parking gear becomes.
In such a situation, if the changing from the parking range P to another transmission range is to be performed, a large force is needed in order to draw the claw of the parking lock pawl from the parking gear since the claw is strongly engaged into the parking gear as mentioned above.
Against this inconvenience, the aforementioned related art is constructed so as to change the transmission range from the parking range P to another range in the above-described situation by forcibly drawing the claw of the parking lock pawl from the parking gear through a power assist operation via a motor or the like.
If the construction in which the parking engagement is forcibly released is adopted, an excessive load can be applied to the parking gear or the claw of the parking lock pawl, and therefore, there can arise a problem of increased deformation of the parking gear or the claw of the parking lock pawl.
Furthermore, in the case where a vehicle tows a trailer, such as a camping trailer or the like, the above-described situation results in a further increased parking load than in the case where a vehicle is used alone without a trailer. Therefore, a vehicle towing a trailer can experience an instance where the changing from the parking range P to another transmission range is very difficult.
Therefore, if a vehicle towing a trailer should fall into the above-described situation, great amounts of labor and cost can be needed, that is, large-scale measures can need to be taken; for example, the trailer needs to be separated from the vehicle, or the vehicle needs to be lifted and forcibly moved from the road by a wrecker or the like, etc.